As indicated above, the present invention relates to methods and apparatus for removal of organic solvents from various aqueous process streams, which are employed in many industrial applications. The invention will be described in detail in connection with a copper solvent extraction/electrowinning process which is currently in use, but it will be recognized by those of skill in the art that the invention has applicability beyond this particular environment.
Modern techniques for recovery of copper from ore involve the use of organic solvents to transfer the copper from a first weakly acidic aqueous process stream to a second strongly acidic aqueous solution via ion exchange. In a first step, a first copper-rich, weakly acidic aqueous solution, referred to as a "pregnant leach" solution, is mixed with the organic solvent. The copper ions are preferentially bound to the solvent. The solvent is immiscible in the aqueous solution and can be separated therefrom by settling. The organic solvent, having had the copper ions bound thereto, is then mixed with a strongly acidic "electrolyte" solution. In this step, the copper is removed from the solvent by the electrolyte, while the molecules of the organic solvent pick up a hydrogen ion from the sulfuric acid. The solvent is then physically separated from the electrolyte by settling and is reused. The copper is subsequently removed from the electrolyte by electrowinning, that is, by electroplating the copper out of the electrolyte solution onto a cathode in a plating bath.
The electrolyte, which is an aqueous solution containing a relatively high proportion of sulfuric acid, the leaching solution, which is an aqueous solution containing a relatively low amount of sulfuric acid, and the organic solvent are thus all reused in the process; the process does not "consume" any of these materials. Electricity is the primary source of energy which is applied.
However, the degree to which these materials can be reused depends on the degree to which they are separated from one another at several stages in the process. More particularly, after the leaching solution has been exposed to the ore, becoming a copper-rich "pregnant leach" solution, it is then mixed with the organic solvent, which preferentially exchanges its hydrogen ions for the copper ions in the solution. As noted, because the organic solvent is immiscible with the aqueous solution, it can largely be separated therefrom by settling. However, some fraction of the organic solvent is inevitably entrained with the aqueous solution, and is lost unless it is separated therefrom. Similarly, some fraction of the organic solvent inevitably remains entrained with the electrolyte after they are substantially separated by settling. Such organic solvents are extremely expensive and are used in extremely large quantities in industrial-scale applications of this technique, such that it is highly desirable that efficient separation be provided.
The prior art suggests several techniques which might be employed for processing aqueous solutions to remove entrained organic solvents. For example, additional settling steps can be performed, or the aqueous solution can be filtered. However, filtration requires large vessels and extremely large quantities of filtration media, due to the enormous quantities of aqueous solution which must be processed. Settling is effective, but requires extremely large tanks, again due to the large volumes of the aqueous solutions which must be processed in any industrial-scale application of this technique. Indeed, most settling tanks are so vast that they must necessarily be exposed to sunlight, the ultraviolet radiation in which causes many organic solvents to break down. Accordingly, neither known technique is cost effective for removing entrained organic solvents from an aqueous process stream.
It is the understanding of the present inventors that techniques have been tried in which air has simply been mixed with the aqueous solution in settling tanks in order to remove the entrained organics, but that these methods have been unsuccessful in large-scale applications.